someone972

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Occasionally the idle on my car is unusually high, around 1,250 RPM in park. Turning the key off and back on brings it back down to where it should be. Could this be an IAC issue perhaps? I never did the IAC learn after having the battery unplugged while replacing the EGR, but I believe it happened before then as well. Anyone run into this before?

mmadden55

Check to see if it is throwing any codes.

Bob88GTA

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Follow this to a T

https://www.thirdgen.org/newdesign/tech/tpimod2.shtml

mmadden55

good info Bob.

someone972

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So I checked the TPS sensor, it might be a bit low at WOT (ECM reading below 4 volts at WOT) but idle is good (around 0.61 usually). The idle speed is fine as well, and I even did the idle-relearn after disconnecting the battery. It doesn't seem to have fixed the problem though. It's only every so often that it happens, most of the time the idle is just fine. But on an occasional trip it will idle too high. It usually starts out like a normal trip with normal idle, but then after some driving I come to a stop and the idle stays higher than it should and stays that way until I key-off key-on. I have a spare 730 ecm, should I try popping it in and see if it makes a difference?

Bob88GTA

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someone972, Maybe pull the IAC out and cleaning the carbon off it and maybe the IAC passage. It may be carbon-ed up. Then do the reset ALL OVER AGAIN! My 88 had that issue also. Finally bought a AC Delco IAC and it remedied the problem.

someone972

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Recently replaced the IAC and tried to clean it out then, but I suppose it wouldn't hurt to give it another shot. It does sound like a sticking IAC problem.

someone972

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Ok, an update. I cleaned out the IAC passage real good with some cleaner and a brush with plastic bristles. Got a reasonable amount of black material out and redid the relearn procedure. Seemed to be running good for a day or two but today the problem came back. Here's a breakdown of what happens:

-Cold start car. It's about 60 degrees today. No idle issues.
-Take off for work. No idle issues while it's warming up.
-Car is fully warmed up. Start entering stop and go traffic due to many red lights.
-When stopped, idle begins hunting. Will drop to where it's supposed to be, feel a little rough, then go up 1-2 hundred rpm, sit there for a second or two and drop down for a little under a second and repeat.
-By the time I get to workplace and put it in park the idle is sitting steady at 1K rpm, usually is at 700 or so.
-Turn car off, immediately turn car back on, idle shoots up higher than it usually does on key on but then settles back down to around 700 like it should.

I'll try to get a datalog of the occurrence, but I usually don't have my laptop with me when it happens. In the meantime, any more ideas or things to test?

keykey

I just today discovered that I have the same "intermittent high idle" problem that you have. A week ago I replaced my ECM with a new reman one, which fixed several problems I had. A couple of issues i had was that the coolant temp was reading wrong, TPS reading never went above 2 volts even with pedal at WOT, and i had the same "hunting idle" issue as you. Those problems are gone now, but the high idle issue is still there.

Based on that, I think your "hunting idle" and "high idle" problems might be two separate issues as that's the case with my car.

Check your 5V sensor circuit by doing the diagnosis for the TPS that is in the factory manual, that's how I found out my ECM was shot. You are supposed to jump two pins together and then jump one pin to battery voltage with a test light, that's supposed to simulate TPS at WOT for the ECM, and it should show 100%, but my only showed 20% because the 5V circuit was bad.

I first found out that something was wrong by connecting my ALDL cable and the coolant temp was reading 40 degrees C when it was 5 degrees C outside and the car had been sitting untouched for days. Then I looked in the wiring diagrams and found out that the 5V circuit is shared with the TPS.

someone972

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Alright, I finally got a datalog of the event. It looks like the TPS for some reason starts reading too high causing the ECM to think it's above 0% at idle, which is causing the IAC to stay open. TPS sweeps fine through it's whole range and goes from 0.59-0.61 ish Volts to 4+ Volts with no hiccups. I've attached the log, around 18:04 is a good example of it (it's at a stop, so look at vehicle speed being 0). Anyone got any idea why the reading changes sometimes? The cable isn't binding, I know that much at least.